High-density carbon--carbon composites are extremely strong materials capable of withstanding high temperatures. When compared with superalloys and ceramics, carbon--carbon composites alone exhibit the unusual property of maintaining strength at temperatures up to at least 4000.degree. F. (2205.degree. C.). Other attributes include
thermal stability as a solid; PA1 high resistance against thermal shock, due to high thermal conductivity and low thermal expansion behavior; and PA1 high strength and stiffness in high temperature application. PA1 R.sup.1 and R.sup.2, which may be the same or different, are C.sub.1 -C.sub.6 alkyl, C.sub.2 -C.sub.6 alkenyl, C.sub.2 -C.sub.6 alkynyl, C.sub.3 -C.sub.7 cycloalkyl, C.sub.5 -C.sub.7 cycloalkenyl, aryl, (C.sub.1 -C.sub.6 alkyl)-substituted aryl, (C.sub.2 -C.sub.6 alkenyl)-substituted aryl, or (C.sub.2 -C.sub.6 alkynyl)-substituted aryl; PA1 R.sup.3 and R.sup.4, which may be the same or different, are H or C.sub.1 -C.sub.6 alkyl; and PA1 R.sup.5 is H or C.sub.2 -C.sub.6 alkynyl.
In view of these attributes, applications of such materials include their use as supersonic aircraft components, their use as the mold material in hot molding in powder metallurgy, and their use in nuclear reactors in the construction of high temperature heat exchangers.
Carbon--carbon composites exhibit only one major disadvantage--a tendency to undergo high temperature oxidation, forming carbon monoxide and carbon dioxide at about 500.degree. C. in an oxidizing atmosphere. Numerous attempts have been made to overcome this tendency. Included among these are the use of pack cementation of silicon carbide as a primary oxidation barrier. Differences in the thermal expansion behavior between the silicon carbide coating and the carbon--carbon composite substrate, however, cause cracks to form within the silicon carbide layer upon exposure of the material to the temperature cycling normally encountered in use. Such cracks allow exposure of the carbon substrate to air, thereby permitting the high temperature oxidation to occur.
In an attempt to eliminate this problem, secondary protective systems have also been developed. Examples are the use of crack sealants added to the composite in particulate form. The crack sealants are borate glasses which seal cracks in the outer silicon carbide coatings as they form. Even with the crack sealants, however, the coatings are useful only in those applications where the temperature remains below the coating fabrication temperature. In addition, the borate glass is incompatible with the carbon--carbon matrix resin during processing, and is moisture sensitive as well, causing blistering, blooming and delamination, and thus precluding its use in a practical system.
Further attempts to overcome oxidation include the use of oxidation inhibitors in the form of inert particulate solids incorporated in the carbon--carbon composite as fillers. A typical composite will contain 60 parts resin to 40 parts inhibitor. The disadvantage of these inhibitors is that they are detrimental to the properties of the composite. They act as a solid barrier to compaction of composite ply layers during processing, and thus increase the spacing between the ply layers. This, combined with the amount of resin displaced by their presence, leads to significant reductions in such interlaminar properties as in-plane and interlaminar tensile strength.